Patients with acute pyleonephritis may present with a spectrum of clinical symptoms and signs. There are few noninvasive diagnostic studies, however to confirm or exclude this diagnosis. To evaluate the clinical utility of 99mTc-DMSA renal scan in diagnosis of acute pyelonephritis, we performed 99mTc-DMSA renal scan in 37 patients suspected with urinary truct infection. Simultaneously kidney ultrasonography was done in 21 patients diagnosed with acute pyelonephritis, clinically. And we performed the followup scan after treatment in two to six weeks. The results were as follows. 1) 99mTc-DMSA renal scan disclosed single of multiple cortical defects and decreased radiouptake ratio of affected kidney in 23 patients among 25 patients diagnosed with acute pyleonephritis. 2) In the 21 patients with acute pyelonephritis, kidney ultrasonography showed abnormal findings in the 7 patients. And 99mTc-DMSA renal scan disclosed abnormal cortical defects in this 7 patients also. 3) Between the patients with acute pyelonephritis and those with lower urinary tract infection, asymptomatic bacteriuria or pyuria, right-to-left radiouptake ratios (R/L ratio ) were significantly different(p<0.001). 4) In two to six weeks after antibiotic therapy, we performed followup 99mTc-DMSA renal scan for 5 patients among 25 acute pyelonephritis patients. And we have found the improvement of cortical defects and the right-to-left radiouptake ratio. In conclusion, we thought that 99mTc-DMSA renal scan should be useful in diagnosis of acute pyelonephritis and follow up examination.